union -containers -package
The union function returns the list union of the two lists. For example,
> "dog" `union` "cow" == "dogcw"
Duplicates, and elements of the first list, are removed from the the second list, but if the first list contains duplicates, so will the result. It is a special case of unionBy, which allows the programmer to supply their own equality test.
The unionBy function is the non-overloaded version of union.
Combines the two file modes into one that contains modes that appear in either.